1. Field of the Invention
The prevent invention relates to an OFDM receiving apparatus that receives an OFDM signal using a plurality of OFDM branches.
2. Description of the Related Art
As a digital-signal transmission system, OFDM (Orthogonal Frequency Division Multiplexing) has been proposed and implemented in recent years. In the OFDM system, data is transmitted employing a plurality of carriers that are orthogonal to each other in the frequency domain. For this reason, the symbol period of data transmitted using each of the carriers is longer according to the OFDM system, resulting in less degradation of reception quality even in a multipath environment with large delays. In addition, a different demodulation method can be selected for each of the carriers.
The modulation performed according to the OFDM uses IFFT (Inverse Fast Fourier Transform), and the demodulation uses FFT (Fast Fourier Transform). Therefore, the OFDM system has high frequency efficiency, and its application to digital terrestrial broadcasts has been widely explored. In Japan, the digital terrestrial broadcasting system called ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) has adopted the OFDM.
In the digital terrestrial broadcasting, signals are often received with a mobile station such as a cellular phone terminal. Therefore, diversity reception has been practically used in order to improve the reception quality. Using diversity reception, a plurality of antennas receive identical signals, and the plurality of received signals are combined to remove noises. Alternatively, there is also a known method that involves selecting, from the plurality of received signals, a signal that has been received through an antenna with better reception condition.
FIG. 1 is a diagram showing a general configuration of an OFDM receiving apparatus equipped with diversity reception function. The OFDM receiving apparatus shown in FIG. 1 has two OFDM branches.
Each OFDM branch has an antenna 101. In each OFDM branch, an OFDM signal is received by a tuner 102 and converted into a digital signal by an A/D conversion unit 103. An orthogonal demodulation unit 104 generates orthogonal signals (an I-component signal and a Q-component signal) from the digital signal obtained in the A/D conversion unit 103. An FFT unit 105 converts the signals from the time domain to the frequency domain, by performing FFT calculation for each symbol. A transmission path equalization unit 106 corrects phase rotation occurred in the transmission path.
A diversity combining unit 107 combines a pair of signals output from the transmission path equalization unit 106 in each OFDM branch. The signals are combined in the frequency domain. For this reason, such diversity reception is also called frequency diversity. In addition, selection diversity can be applied instead of the diversity combining. An error correction unit 108 performs error correction for the output signal from the diversity combining unit 107, thereby regenerating the transmitted data.
In the diversity combining configured as described above, output Y of the diversity combining unit 107 is expressed by the equation shown below. In the equation, D1 and D2 represent data output from the transmission path equalization unit 106 in each OFDM branch. P1 and P2 represent power of the SP carrier in each OFDM branch.Y=(P1*D1+P2*D2)/(P1+P2)
Diversity combination makes it possible to reduce correlation between branches to improve reception sensitivity, by adjusting, for example, antenna directivity. However, since signals are received from a plurality of paths, severe deterioration in the quality of received signals in one branch could lead to signal degradation in other branches.
An OFDM receiving apparatus having diversity reception function often adopts the master-slave system. An OFDM receiving apparatus adopting the master-slave system is described below, referring to FIG. 2A and FIG. 2B. FIG. 2A and FIG. 2B show the front stage and the subsequent stage of the OFDM receiving apparatus, respectively. The FFT unit is included in both FIG. 2A and FIG. 2B.
As shown in FIG. 2A, a clock recovery unit 111 in each branch recovers the clock for the output signal from the orthogonal demodulation unit 104. A guard correlation unit 112 detects the phase error for each symbol. A phase error averaging unit 113 calculates the average value of the phase errors detected by the guard correlation unit 112. A clock correction unit 114 corrects the error of the clock recovered by the clock recovery unit 111, using the average value of the phase errors. Thus, in an OFDM apparatus adopting the master-slave system, basically the clock is recovered independently at each branch and corrected separately at each branch.
In each branch, the FFT unit 105 and the transmission path equalization unit 106 operate in accordance with the clock recovered in the respective branch. The diversity combining unit 107 operates basically using the clock in either one of the branches. For example, assuming that the first branch is the master, the diversity combining unit 107 operates using a clock 1 recovered in the first branch. For this reason, a clock conversion unit 115 needs to be provided for other branches to perform clock conversion. The clock conversion unit 115 is realized, for example, by implementing a memory to store data, and setting a write clock and a read clock that are different from each other.
Patent Document 1 (Japanese Patent Application Publication No. 2006-50283) describes a configuration in which symbol positions of a first branch and a second branch are detected respectively, and clocks to be used by A/D conversion units of the branches are generated in accordance with the pair of the detection results. In the configuration, a carrier-frequency error correction unit performs the correction separately for each branch.
According to the master-slave system described above, diversity operations are performed in accordance with the clock in the master branch. For this reason, when quality deterioration occurs in the master branch and frame synchronization becomes ineffective, transmitted data cannot be regenerated even if the slave branch maintains good quality. A possible way to solve the problem would be, for example, to employ a configuration in which quality of each branch is monitored and a branch with higher quality is set as the master (in other words, diversity operations are performed using the clock of the branch with higher quality). However, the configuration requires complicated configuration and operations to switch the clocks. In addition, a steep decline of the quality of the master branch causes delay in clock switching and the like, which makes it impossible to receive signals.
According to the configuration described in Patent Document 1, the clock for each branch is recovered on the basis of information detected in each branch. For this reason, if the configuration described in Patent Document 1 is applied to a circuit for correcting a clock recovered from a received OFDM signal, a steep decline in quality of a branch would cause quality of other branches to decline as well. In addition, reception characteristics also deteriorate, when the phase error at each branch largely fluctuates due to factors such as a multipath environment, or when the control method for the FFT window is changed because of, for example, presence of a preceding wave.